High-speed counters from which the count value can be reliably extracted are useful in such systems as radar range measuring equipment, multiple time interval measurement systems, frequency monitoring systems, and the like. In most such equipment, accuracy improves with the speed of the counter. When using high-speed counters, however, it is difficult to read out the value in the counter without disturbing its operation or obtaining a wrong value. The count extraction scheme tends to become more complex when the input pulses being counted are not evenly spaced.
To extract the count between input pulses is difficult for two reasons. The storage registers must be as fast as the fastest counting stage, viz., the least significant bit. Furthermore, the ripple carry is liable to cause an error at some stage because its location can not be ascertained. This second difficulty can be somewhat alleviated by using a synchronous counter, but this makes the counter more complex and expensive. It is not usually feasible to segment the synchronous counter because this again introduces an unpredictable propagation delay between the segments.
A reliable count extraction is one in which there is no error (or anomaly) or, if there is an error, it is known.
The invention disclosed herein describes a high-speed counter having a count extraction means which does not interrupt the counting of the counter, uses few high speed stages, and resolves any anomolies in the extracted count value.